1. Field of the Invention
The present invention relates to a superconductive coil assembly of the internal cooling type and more particularly, it relates to an improvement of the reinforcing vessel for the superconductive coil assembly.
2. Description of the Related Art
The forcedly cooled superconductive coil assembly of the internal cooling type is incorporated into the magnetic confinement fusion apparatus as the one such as Troidal and Poloidal coils to which a large current is supplied. FIG. 1 shows the conventional superconductive coil assembly of this type. Plural sheath tubes 1 each having a rectangular section and a hollow portion through which superconductive wires 2 are inserted and which serves as passage 3 for allowing supercritical helium to flow therethrough are prepared. Insulating layer 4 is formed on the outer surface of each of sheath tubes 1. These sheath tubes 1 are bundled and enclosed by insulating layer 5. In the case of this coil assembly, sheath tubes 1 serve as boundaries against the pressure of supercritical helium. Even when superconductivity is lost or superconductive coils are quenched to raise pressure to a great extent in sheath tube 1, sheath tube 1 serves to accommodate this pressure. Further, it serves to support matters such as superconductive wires 2 relative to electromagnetic force generated when current flowing through the superconductive matters electromagnetically interferes with the matters. It also serves to support the superconductive matters relative to magnetic field caused by the superconductive matters. Therefore, the superconductive coil assembly is held stable by sheath tube 1.
When a large current is intended to flow through superconductive wires 2 as seen in the case of the Toroidal or Poloidal coil in the superconductive coil assembly having the above-described arrangement, the sectional area of superconductive wire 2 is set large and pressure in sheath tube 1 is raised to a substantially large value because of quenching. In order to sufficiently withstand this inner pressure, therefore, it is needed that the wall of sheath tube 1 is made thick. When the wall of sheath tube 1 is made thick, however, the shaping of sheath tube 1 at the process of making sheath tube 1 rectangular and winding it in a spiral in the course of producing the superconductive coil assembly becomes extremely difficult. In addition, undesirable force may be added to superconductive wires 2 to break them while sheath tube 1 is being shaped. It is sometimes seen that sheath tube 1 is deformed like a trapezoid, as shown in FIG. 2, by bending pressure added to sheath tube 1 while sheath tube 1 is being shaped like a spiral. When sheath tube 1 is deformed like this, it is difficult to produce the coil assembly of sufficiently high accuracy. In addition, there is a fear that insulating layer 4 by which the outer surface of sheath tube 1 is wrapped is broken by this deformation of tube 1. When insulating layer 4 is not broken but electromagnetic force acts on sheath tube 1, adjacent sheath tubes 1 are not plane-contacted but point-or part-contacted with each other, thereby making it likely to lose insulation. When the wall of sheath tube 1 is thick, the quantity of heat caused at the time of air-tightly welding sheath tube 1 becomes larger and larger as the wall of sheath tube 1 is made thicker and thicker, thereby causing superconductive wires 2 to be broken.
As described above, the wall of sheath tube 1 is made thick in the case of the conventional superconductive coil assembly. Therefore, the shaping of sheath tube 1 in a coil becomes difficult and the deforming thereof is likely to be caused. The superconductive wires are broken and the dimensional accuracy of the superconductive coil assembly is reduced. Insulation is likely to be lost because the insulation layer for insulating sheath tube 1 from the other adjacent ones is damaged. The superconductive wires may be broken by the increase of heat quantity created at the time of welding the sheath tube.